docs: def mean_by_panel(self, length):
          """
          Compute the mean across fixed sized panels of each record.
  
          Splits each record into panels of size `length`,
          and then computes the mean across panels.
          Panel length must subdivide record exactly.
  
          Parameters
          ----------
          length : int
              Fixed length with which to subdivide.
          """
          self._check_panel(length)
          func = lambda v: v.reshape(-1, length).mean(axis=0)
          newindex = arange(length)
          return self.map(func, index=newindex)